1. Field of the Invention
The present invention relates generally to light sources used in optical communication systems, and particularly to laser light sources capable of generating steady light with controlled intensity.
2. Description of Related Art
Semiconductor lasers are used in many fields including optical communication systems and optical memory systems. Semiconductor lasers include edge-emitting lasers and surface-emitting lasers. Edge-emitting lasers require high threshold current, therefore surface-emitting lasers are frequently preferred. In particular, vertical-cavity surface-emitting lasers are being adopted in many new systems because they are relatively easy to manufacture and are cost-efficient. Such surface-emitting laser must be mounted in a package to prevent dirt and external temperature variations from impairing the performance of the laser, and to prevent components within the package from aging prematurely. The package provides a stable working environment for the laser, and the light intensity of the laser can be maintained at a steady level.
FIG. 1 shows a conventional method for packing a laser adopting TO-CAN structure, as disclosed in U.S. Pat. No. 5,835,514. A light source 1 comprises a header 4, electrical conductors 5, 6 and 7, a light sensor 8, a laser 9, a can 2 and a convex beam-splitting lens 11. The light sensor 8 is mounted on the header 4. The laser 9 is mounted on the light sensor 8, with a light receiving area of the light sensor 8 surrounding a periphery of the laser 9. A window 10 is mounted in a top portion of the can 2. The can 2 is mounted on the header 4, and the laser 9 and the light sensor 8 are mounted within the can 2. The convex beam-splitting lens 11, such as a ball lens, is mounted within the window 10. The electrical conductors 5, 6 and 7 electrically connect with and extend from the header 4 to connect with an external control circuit. Thus, the laser 9 and the light sensor 8 are connected with the external control circuit. Part of the light beam generated by the laser 9 is reflected by the convex beam-splitting lens 11 to the light sensor 8. The light sensor 8 generates an electrical signal in response to the reflected light beam, and transfers the signal to the external control circuit via the electrical conductors 5, 6 and 7. The light intensity of the laser 9 is thereupon adjusted to maintain the emission power of the laser 9 at a steady level.
The above-described light source has serious drawbacks. The convex beam-splitting lens 11 is a ball lens. Thus, the reflected light beam is scattered over a wide area including areas outside a periphery of the light sensor 8. Furthermore, the laser 9 is mounted on the light sensor 8, thereby blocking part of the light receiving area of the light sensor 8. These factors in combination result in only part of the reflected light beam being received by the light sensor 8. Furthermore, part of the reflected light beam 14 goes back to the laser 9 itself. This interferes with the light being emitted by the laser 9, and results in unsteady emission of the laser 9.
FIG. 2 is a graph showing the relationship between emission power P1 and the drive current I of the laser 9 at different external temperatures T1, T2. It can be seen that the P1-I characteristic curve 12 changes into the P1-I characteristic curve 13 in response to a change of the external temperature from T1 to T2. That is, at an identical drive current, the laser 9 generates different emission powers in response to different external temperatures. In order to maintain the emission power of the laser 9 at a steady level, an external control circuit needs to be adopted to adjust the drive current to the laser 9 and thereby control the emission power of the laser 9.
Referring to FIG. 3, curves 23, 24 respectively show the relationships between reflected light power P2 of the laser 9 and the light receiving area X1-X2, X3-X4 of the light sensor 8 at different external temperatures T1, T2. That is, the light energy distribution characteristic curve 23 of the light receiving area X1-X2, X3-X4 of the light sensor 8 changes into the characteristic curve 24 in response to the change of the external temperature from T1 to T2. A ratio of the light energy received by the light sensor 8 to the total reflected light energy is variable. Accordingly, the light sensor 8 inaccurately measures the output light power of the laser 9. The external control circuit cannot control the laser 9 precisely, and the emission power of the laser 9 is unstable.
FIG. 4 shows another conventional method for packing a laser, which includes an inclined top structure. A laser 19 and a light sensor 20 of a light source 111 are separately mounted on a header 17. A planar beam-splitting lens 22 is placed within a window 21 of an inclined top of a can 15. This design eliminates the problem of reflected light going back to the laser 19 itself. However, the beam-splitting lens 22 does not converge the reflected light beams. The light sensor 20 still receives only a fraction of the reflected light beams, which results in imprecise measurement of a ratio of the reflected light to the emitted light. Furthermore, the beam-splitting lens 22 is sensitive to changes in temperature. The beam-splitting lens 22 distorts with any change in temperature, resulting in a variable spread of reflected light beams reaching the light sensor 20. Thus, the external control circuit cannot precisely control the light intensity of the laser 19, and the emission power of the laser 19 is unstable.
Referring to FIG. 5, curves 25, 26 respectively show the relationships between reflected light power P3 of the laser 19 and the light receiving area X1-X2 of the light sensor 20 at different external temperatures T1, T2. That is, the light energy distribution characteristic curve 25 of the light receiving area X1-X2 of the light sensor 20 changes into the characteristic curve 26 in response to the change of the external temperature from T1 to T2. A ratio of the light energy received by the light sensor 20 to the total reflected light energy is variable. Accordingly, the light sensor 20 inaccurately measures the output light power of the laser 19. The external control circuit cannot control the laser 19 precisely, and the emission power of the laser 19 is unstable.
In view of the above, it is an object of the present invention to provide a light source, the emission power of which can be maintained at a steady level.
It is another object of the present invention to provide a light source having a beam splitter which can converge light beams to obviate the effects of changes of the external temperature.
In order to achieve the objects set above, a light source in accordance with the present invention generates an output light beam having a controlled intensity. The light source comprises a header, electrical conductors, a light sensor, a laser, a can, a convex lens, and an external control circuit. A window is defined in an inclined top of the can, and a convex lens serving as a beam-splitter is mounted within the window. A convex surface of the inclined convex lens faces away from the light sensor. The laser and the light sensor are separately mounted on the header. The electrical conductors extend from the header to electrically connect with the external control circuit. Emitted light beams of the laser are divided by the convex lens into output light beams and reflected light beams. The output light beams are transmitted through the convex lens as an optical carrier wave for signals. The reflected light beams are reflected toward the light sensor as feedback light for measurement. The light sensor generates electrical signals representing the intensity of the light energy falling on it. The external control circuit adjusts the emission power of the laser according to the electrical signals it receives from the light sensor. The inclined convex lens has a pre-determined curvature such that emitted light beams of the laser reflected from the lens converge onto the light sensor. Accordingly, the reflected light beams received by the light sensor are accurately proportional to the emitted light power of the laser. Thus the external control circuit can precisely control the light intensity of the laser to maintain the emission power of the laser at a steady level.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: